1. Field of the Invention
The present invention relates to a technology for allocating an appropriate label to a working route and a standby route in a ring network with a function of ring protection.
2. Description of the Related Art
Conventionally, in an Internet Protocol (IP) network, a transfer destination of an IP packet is determined based on a result of a calculation between an IP address and a netmask. However, the processing load on a router increases along the increase in the transmission speed in the network and along the increase in the quantity of IP packets processed by the router per unit time.
Thus, a new routing system called a Multi-Protocol Label Switching (MPLS) is recently widely used. According to the MPLS, address information called a label is attached to an IP packet, and a transfer destination of the IP packet is determined based on the label. Therefore, a routing can be achieved without carrying out the calculation, thereby avoiding the increase in the processing load on the router due to the increase in the transmission speed in the network.
When the MPLS is used, a path needs to be established between routers that carry out communications, and a label needs to be allocated to the path. When the network has a small scale, an administrator can establish a path and allocate a label. However, when the network has a large scale, load on the administrator becomes very heavy. Thus, a signaling technique such as a Resource Reservation Protocol-Traffic Extension (RSVP-TE), which makes each router autonomously establish a path and allocate a label, is drawing attention. The RSVP-TE is described in D. Awduche, L. Berger, D. Gan, T. Li, V. Srinivasan, G. Swallow “RSVP-TE: Extensions to RSVP for LSP Tunnels”, December 2001, URL: http://www.ietf.org/rfc/rfc3209. txt.
The MPLS that makes a router efficiently carry out a routing can also be used in a network other than the IP network. A Generalized Multi Protocol Label Switching (GMPLS), which is a generalized MPLS, can be used in a network using optical fibers such a synchronous optical network (SONET) and a wavelength division multiplexing (WDM). However, the RSVP-TE and other conventional signaling techniques are based on a mesh structure like that of the IP network. Accordingly, these signaling techniques cannot support a protection function of a ring network such as the SONET.
For example, the SONET has a protection function called a Bidirectional Switched Ring (BLSR), which enabling a communication service to be maintained even when a failure occurs in any part of the ring. The function also becomes valid in the GMPLS when a specific constraint concerning a label allocation is satisfied. However, the RSVP-TE and other conventional signaling techniques cannot satisfy the constraint. Therefore, a path established based on these signaling techniques cannot be protected by the protection function, and the path becomes vulnerable to failure.